Apparatus for absorption of electromagnetic energy reflected from a dense plasma



1966 e. L. HOLLINGSWORTH 3,

APPARATUS FOR ABSORPTION OF ELECTROMAGNETIC ENERGY REFLECTED FROM ADENSE PLASMA Filed March 25, 1964 INVENTOR. gull/ ORD Ll/OLL/A/GSWo/FT/l Arr).

United States Patent APPARATUS FOR ABSORPTION OF ELECTRO- MAGNETICENERGY REFLECTED FROM A DENSE PLASMA Guilford L. Hollingsworth, Seattle,Wash, assignor to The Boeing Company, Seattle, Wash., a corporation ofDelaware Filed Mar. 25, 1964, Ser. No. 354,722 12 Claims. (Cl. 343-18)This invention relates to the transmission of electromagnetic energythrough a dense plasma medium and more particularly to the removal ofelectromagnetic energy reflections caused by a dense plasma medium so asto prevent physical damage to a transmitter of said electromagneticenergy by said electromagnetic energy reflections.

The teachings of this invention involve subject matter related to acopending application Serial No. 346,792, filed February 24, 1964, andentitled Apparatus for Eifecting the Transmission of ElectromagneticEnergy Through a Dense Plasma; invented by Melvin I. Kofoid. Inparticular, an antenna window wall is used to achieve transmission ofelectromagnetic energy through a dense plasma medium in a mannerdescribed in the above-mentioned copending application; However, theteachings herein refer particularly to an antenna wall constructionwhich presents a constant impedance when viewed from one side, i.e., thetransmitter of electromagnetic energy side of the antenna window wall.

A typical situation in which the instant invention derives utility canbe visualized by considering radio transmission from a high speedvehicle to a distant receiver. A layer of ionized gas commonly presentabout such a vehicle can cause essentially total reflection oftransmitted radio signals, i.e., minimal transmission. As taught in thecited copending application, a window through such a dense plasma can beopened so as to effect almost total transmission of radio waves.However, additional reflection oftransmitted energy may occur fromreflecting media at some distance from the high speed vehicle; suchreflected energy can cause damage to a transmitter where it is permittedto impinge thereon. It is therefore a feature of the instant inventionto remove these last mentioned energy reflections before they impingeupon the transmitter.

The problem of removing reflected electromagnetic energy before damageto a transmitter results has met with varied solutions. For example, theuseof the wellknown gyrator which effectively swallows, so to speak,reflected electromagnetic energy before such reflections can damage atransmitter. I

As taught in the above-cited copending application an antenna cover wallwill provide a strong magnetic field in proximity to a juxtapositioneddense plasma medium so as to effect almost total transmission, throughthe dense plasma, of electromagnetic energy. The antenna cover walleflectively eliminates reflections of electromagnetic energy on passagethrough the wall itself. However, the problem of reflection from areflective medium located at a distance from the antenna cover wall mustbe controlled in order to prevent damage to the transmitter. Theteachings of this invention provide a solution to this problem in amanner that is superior to others in the art from both an economy ofweight and space concept, while maintaining high efficiency.

As set forth in the above-mentioned cop-ending application, an antennacover wall which incorporates a ceramic magnetic material sandwichedbetween and bonded to two dielectric sheets M4+k()\/2) in electricthickness, where k is zero or any whole number, will effectuate trans-3,295,131 Patented Dec. 27, 1966 mission of electromagnetic energythrough the antenna cover wall with minimal attenuation andsubsequently, with the same result, through a juxtapositioned denseplasma medium. As mentioned in the referred to copending application,once dielectric sheets of prescribed electric thickness are placedagainst a ceramic magnetic material and all materials are chosen to havea certain permeability constant and dielectric constant relationship,energy reflections are minimized regardless of the effect the ceramicmagnetic material may have in causing angular rotation of the electricfield vector of the electromagnetic energy. However, should thistransmitted electromagnetic energy encounter additional dense plasmamedia at some distance outside the effect of the ceramic magneticmaterials field, reflections of electromagnetic energy from the lattermentioned dense plasma media will occur and result in a net energyreflection arriving back at the transmitter.

Consider, therefore, an antenna cover wall of the type mentioned abovebut having a ceramic magnetic material of a type that will elfect aone-way angular rotation of the electric field vector of electromagneticenergy equal to 45 degrees. That is, instead of a wall which causes anangular rotation of some arbitrary amount, we choose a ceramic magneticmaterial that will rotate, angularly, the electric field vector ofelectromagnetic energy 45 degrees as the electromagnetic energy passesone way through the antenna cover wall. Therefore, any energy which isreflected back towards the transmitter will pass again through theantenna cover wall and have experienced a total angular rotation of itselectric field vectorof degrees. This reflected energy'is effectivelyremoved before it-damages the transmiter by providing for a matchedconstant impedance on the transmitter side of the antenna cover wall.Therefore, in connection with the above antenna cover wall considerenergy absorbing means such as several sheets of dielectric coatedresistive material, having their surface planes parallel to one anotherand disposed between the transmitter of electromagnetic energy and theantenna cover wall. By transmitting electromagnetic energy so that theelectric field vector is normal and non-coplanar to the above absorbingmeans, any reflection of the electromagnetic energy after passingthrough the antenna cover wall and again encountering the absorbingmeans will have had the electric field vector rotated 90 angulardegrees, as discussed above, so as to now lie co-planar to the absorbingmeans. Absorption of the reflected energy is thereby eifected,preventing reflected energy from impinging upon the transmitter. Theinvention as taught by this embodiment can be practiced by an antennacover Wall which rotates the electric field vector of electromagneticenergy 45 or 45 +k(90) angular degrees as the electromagnetic energypasses one way through the antenna cover wall, where k is any wholenumber. With any of the above angular rotations, however, transmittedelectromagnetic energy must impinge upon the absorbing means with theelectric field vector normal and noncoplanar thereto.

The teachings of this invention are not restricted, however, to the useof a ceramic magnetic material which effectuates angular rotation of anelectric field vector of 45 +k(90) angular degrees, where k is zero orany whole number. Considering the above absorbing means which comprisesheets of resistive material, in order that re flected energy beeffectively absorbed by said means the electric field vector of thereflected energy must be coplanar with the surface plane of the sheet ofresistive material. In the above embodiment then, where the transmittedenergy impinges upon the absorbing means with the electric field vectorat some angle other than the angle representing an electric field vectornormal to the absorbing means, the electric field vector of reflectedenergy passing back through the antenna cover wall will have beenrotated so that the electric field vector is now noncoplanar with theabsorbing means and net energy refiection will reach the transmitter.Therefore, where energy is transmitted with the electric field vector atany angle other than that which is normal to the absorbing means, it isa feature of this invention to adapt the ceramic magnetic material ofthe antenna cover wall so as to again rotate the electric field vectorof reflected energy into coplanar relation to the absorbing means.

Consider, for example, transmitted electromagnetic energy which impingesupon absorbing means such as sheets of resistive material with theelectric field vector 30 angular degrees from the normal, i.e., makingan angle of 60 degrees to the surface plane of the resistive sheets. Inorder that any possible reflections of this electromagnetic energy beprevented from impinging back on the transmitter after passing throughan antenna cover Wall, ceramic magnetic material of a type capable ofrotating the electric field vector 30+k(180) angular degrees in a oneway passage through said antenna cover wall is used where k is zero orany whole number. For example, where k is zero, total rotation of theelectric field vector of reflected energy will be 60 degrees, againplacing the electric field vector co-planar to the absorbing sheets.Thus, the combination of absorbing elements and ceramic magneticmaterial in precise relationship is used so that any net reflection ofelectromagnetic energy arrives at the absorbing means with the electricfield vector coplanar with the absorbing means so as to eliminate saidreflections.

It will be recognized from the above examples that many variedcombinations of absorbing means and antenna cover walls are possiblethrough this invention, to achieve a minimizing of absorption oftransmitted electromagnetic energy and a maximizing of absorption ofreflected electromagnetic energy.

Other embodiments of the invention provide different absorbing means.For example, several parallel metal wires placed at the transmitter sideof the antenna cover wall. Electromagnetic energy with the electricfield vector normal to the longitudinal axis of the parallel metal wiresand passing through an antenna cover wall effecting a 45-degree angularrotation of the electric field vector will be prevented from reflectingback upon the transmitter by the combination of the 90-degree totalangular rotation in traveling a round trip through the antenna coverwall and the absorbing wires.

Again, where transmited energy impinges upon the last mentionedabsorbing means with the electric field vector normal thereto andsubsequently passes through an antenna cover wall which effects angularrotation of the electric field vector of 45+k(90) degrees in a one waypassage through the antenna cover wall, where k is zero or any wholenumber, any reflection of the electromagnetic energy passing backthrough the antenna cover wall will be absorbed by the combination ofelectric field vector rotation and said absorbing means. And, wheretransmitted electromagnetic energy impinges on the last mentionedabsorbing means with the electric field vector at varied angles to thelongitudinal axis of said absorbing means, absorption of reflectedenergy is accomplished within the scope of the teachings set forthabove.

Therefore, it is an object of this invention to effectively eliminatereflections of electromagnetic energy occurring on transmission througha reflective medium before said reflections can interfere with atransmitter of said electromagnetic energy.

Another object of this invention is to effectively eliminate reflectionsof electromagnetic energy occurring on transmission through a reflectivemedium before said refiections can interfere with a transmitter of saidelectromagnetic energy by the use of a constant impedance provided byresistive material.

Other Objects of this invention will be apparent from the followingdescription in which:

FIG. 1 is an illustration of one embodiment of the teachings of thisinvention wherein the absorbing elements comprise parallel sheets ofresistive material aifixed perpendicularly to an antenna cover wallwhich includes: (1) a ceramic magnetic material, chosen to effect apredetermined angular rotation of the space electric vector ofelectromagnetic energy passing therethrough and sandwiched between; (2)two dielectric sheets;

FIG. 2 is an illustration of a second embodiment of the teachings ofthis invention wherein like components of FIGS. 1 and 2 have the samereference characters and wherein the absorbing elements compriseparallel wires of resistive material affixed to an antenna cover wallwhich includes: (1) a ceramic magnetic material, chosen to effect apredetermined angular rotation of the space electric vector ofelectromagnetic energy passing therethrough and sandwiched between; (2)two dielectric sheets; and,

FIG. 3 is an illustration of a third embodiment of the teachings of thisinvention wherein like components of FIGS. 1, 2 and 3 have the samereference characters and wherein the absorbing elements compriseparallel wires of resistive material affixed to an antenna cover wallwhich includes: (1) a ceramic magnetic material, chosen to effect apredetermined angular rotation of the space electric vector ofelectromagnetic energy passing therethrough and bonded to; (2) a singlesheet of dielectric.

Although it is taught in the embodiments of FIGS. 1, 2 and 3 that theabsorbing elements are affixed to the antenna cover wall, it is to beunderstood that operativeness of this invention is not dependentthereon. The embodiments of FIGS. 1, 2 and 3 work equally well when theabsorbing elements are simply disposed between the transmitter ofelectromagnetic energy and the antenna cover wall.

Refer-ring to FIG. 1, a composite antenna cover wall 1 is illustratedcomprising a magnetized ferrite sheet 7 of a type ferrite in which thedegree of angular rotation of the space electric vector ofelectromagnetic energy effected in a one-way passage through saidmagnetized ferrite sheet 7 is 45 degrees. The magnetized ferrite sheet 7has arbitrary electrical thickness and is sandwiched between and bondedto two dielectric sheets 3 which are each M4 in electrical thickness.Hereinafter, whenever M4 is designated as proper electric thickness, itis to be understood that 4 plus any number of half-wave lengths inelectrical thickness is appropriate. The dielectric constant 6 of themagnetized ferrite sheet 7 is equal to the dielectric constant 6 of thedielectric sheets 3 raised to the second power, i.e.3 :Eq.

In combination with the antenna cover wall 1 of FIG. 1 is a radiator ofelectromagnetic energy 10 and parallel absorbing elements 9 afiixedperpendicularly to the plane of the dielectric sheet 3. The dielectricsheet 3 to which the absorbing elements 9 are aflixed is the dielectricsheet 3 which is situated most proximate to the electromagnetic energyradiator 10. The absorbing elements 9 may physically be of any ofseveral forms known to the electrical art; thin sheets of resistivematerial having a dielectric coating for example.

In operation, upon transmitting electromagnetic energy through theantenna cover wall 1 from left to right as shown, there will be noabsorption of electromagnetic energy by the absorbing elements 9 sincethe electric field vector of the electromagnetic energy is transmittednormal to the absorbing elements 9. After the electromagnetic energy hasleft the antenna cover wall 1 and passed through a juxtapositioned denseplasma medium with essentially zero reflection, i.e., minimalattenuation, any reflection of the electromagnetic energy which occursfrom a reflecting medium outside of the influence of the antenna coverwalls magnetic field will pass back through the antenna cover wall 1from right to left as shown and will arrive at the front of the antennacover wall 1, i.e.,

interface 18, with the electric field vector lying coplanar withthe-surface plane of the absorbing elements 9. Thus, electromagneticenergy transmitted with the electric field vector in a plane normal tothe antenna absorbing elements 9 and passing through the antenna coverwall 1 of FIG. 1 from left to right as shown will be prevented fromtraveling fromright to left as shown, i.e., from the interface 18 t o atransmitter 19, by the combination of 90-degree angular rotation of theelectric field vector in traveling a round trip through magnetizedferrite sheet 7 of the antenna cov'er wall 1 and encountering theabsorbing elements 9 at interface 18. Hereinafter, whenever a magnetizedferrite sheet 7 effecting a 45-degree angular rotation of the electricfield vector is specified, it is to be understood that 45+k(90) degreesis appropriate where k is zero or any whole number.

In the embodiment of FIG. 2 the antenna cover wall 1 is exactly the sameas in the embodiment of FIG. 1. As shown in FIG. 2, however, theabsorbing elements 11 comprise parallel metal wires aifixed to thesurface of the dielectric sheet 3 most proximate to the electromagneticenergy radiator 10. Reflected electromagnetic energy will be absorbedand thus prevented from impinging upon the transmitter 19 in the mannerdescribed in FIG. 1; i.e., by the combination of 90-degree angularrotation of the electric field vector in traveling a round trip inmagnetized ferrite sheet 7 of the antenna oover wall 1 and encounteringthe absorbing elements 11 at interface 18.

The embodiment of FIG. 3 discloses an antenna cover wall 6 which differsfrom the embodiment of FIG. 2 only in that a M4 dielectric sheet 3 onthe side of antenna cover wall 6 farthest from the radiator 10 ofelectromagnetic energy has been removed. While the antenna cover wall 1of FIGS. 1 and 2; (1) effects transmission of electromagnetic energythrough the antenna cover Wall and a juxtapositioned plasma with minimalreflection of electromagnetic energy; and (2) provides for constantimpedance as seen from the radiator 10 of electromagnetic energy side ofsaid antenna cover wall 1, the antenna cover wall 6 of FIG. 3 will notprovide transmission of high frequency electromagnetic energy withsubstantially zero reflection of transmitted electromagnetic energy butwill offer a constant impedance load as seen from the transmitting side.However, the antenna cover wall 6 of FIG. 3 has the advantage that if amedium at interface 22 of the antenna cover wall 6 has a dielectricconstant with any value between unity and the square of the dielectricconstant of the magnetized ferrite 7, the worst reflection ofelectromagnetic energy encountered Will be simply that which would bewith air associated with a single interface reflection of the magnetizedferrite 7, i.e., that occurring at interface 17 illustrated in FIG. 1.This feature provided by antenna cover wall 6 is of considerableadvantage when the media at interface 22 is ionized gas existing eitheras the plasma sheath about a high-speed vehicle or produced by breakdownof the air outside of the window when high power electromagnetic energyis transmitted. As pointed out above, the embodiment of FIG. 3 presentsa constant impedance when viewed from the transmitter 19 side of antennacover wall 6. Reflections are thus absorbed at interface 18 by absorbingelements 11 in the same manner as in FIG. 2.

Since numerous changes may be made in the above apparatus and differentembodiments may be made without departing from the spirit and scopethereof, it is intended that all matter contained in the foregoingdescription or shown in the accompanying drawing shall be interpreted asillustrative and not in a limiting sense.

I claim as my invention:

1. A composite mechanical structure disposed to cooperate with aradiator of electromagnetic energy and to effect electromagnetic energytransmission through the composite mechanical structure and subsequentlythrough a juxtapositioned dense plasma medium with a minimum ofattenuation of electromagnetic energy and to absorb electromagneticenergy reflected from a reflecting medium at a distance from thecomposite mechanical structure comprising:

(a) at least one sheet of ceramic magnetic material;

(b) at least one sheet of a dielectric material affixed to said sheet ofceramic magnetic material; and,

(c) energy absorbing elements disposed at said at least one sheet ofdielectric material said at least one sheet of a dielectric materialbeing interposed between said ceramic magnetic material and said energyabsorbing elements in sandwich fashion.

2. Apparatus for elimination of electromagnetic energy reflected from areflecting medium comprising:

(a) a sheet of arbitrarily thick but uniformly thick ceramic magneticmaterial bonded to and sandwiched between;

(b) two dielectric sheets, at least one of said dielectric sheets havingdisposed at its surface a plurality of energy absorbing elements, saidlast mentioned dielectric sheet being most proximate to a radiator ofelectromagnetic energy, whereby electromagnetic energy passes from saidradiator through said apparatus.

3. The combination defined in claim 2 wherein each sheet of saiddielectric material is A/4+k()\/2) in electrical thickness, where krepresents zero or any whole number.

4. The combination defined in claim 2 wherein said ceramic magneticmaterial has a dielectric constant equal to the dielectric constant ofthe dielectric sheets raised to the second power.

5. The combination defined in claim 2 wherein said energy absorbingelements are thin sheets of resistive material, parallel to one anotherand affixed perpendicularly at the surface of a dielectric sheet mostproximate to the radiator of electromagnetic energy.

6. The combination defined in claim 2 wherein said energy absorbingelements include wires of resistive material disposed parallel to oneanother at the surface of a dielectric sheet most proximate to theradiator of electromagnetic energy.

7. Apparatus for elimination of electromagnetic energy reflected from areflecting medium comprising:

(a) a sheet of arbitrarily thick but uniformly thick ceramic magneticmaterial bonded to;

(b) a dielectric material having energy absorbing elements disposed at asurface of said dielectric ma terial said dielectric material beingdisposed between said sheet of arbitrarily thick but uniformly thickceramic magnetic material and said energy absorbing elements in sandwichfashion and said energyabsorbing elements being most proximate to;

(c) a radiator of electromagnetic energy disposed to cooperate with saidapparatus so that electromagnetic energy emitted from said radiatorpasses through said apparatus and through a juxtapositioned plasmamedium with minimum attenuation.

8. The combination defined in claim 7 wherein said dielectric materialis A/4+k()\/ 2) in electrical thickness, where k represents zero or anywhole number.

9. The combination defined in claim 8 wherein said ceramic magneticmaterial has a dielectric constant equal to the dielectric constant ofthe dielectric sheets raised to the second power.

10. The combination defined in claim 9 wherein said energy absorbingelements include metallic wire parallel to one another and affixed to asurface of said dielectric material most proximate to the radiator ofelectromagnetic energy.

11. The combination defined in claim 10 wherein said energy absorbingelements include parallel sheets of resistive material affixed to asurface of said dielectric material most proximate to the radiator ofelectromagnetic energy.

7 8 12. A composite mechanical structure disposed to coenergy andwherein each sheet of said dielectric ma Operate with a radiator felectromagnetic energy and to terial is t/4+kx( \/z) in electricalthickness, where effect electromagnetic energy transmission through thek represents zero or any Whole number.

composite mechanical structure and subsequently through ajuxtapositioned dense plasma medium with a minimum of attenuation ofelectromagnetic energy and to absorb electromagnetic energy reflectedfrom a reflecting medium References Cited by the Examiner UNITED STATESPATENTS at a distance from the composite mechanical structure 2,532,15711/1950 Evans 325-24 X comprising: 2,850,624 9/1958 Kales 32524 (a) atleast one sheet of arbitrarily thick but uniformly 10 3,010,084 11/1961Seidel 333-242 thick ceramic magnetic material bonded to; 3,103,6399/1963 Reggin 33324.2 (b) at least one sheet of a dielectric material,having 3,188,582 6/1965 Bowness 333-24.2

energy absorbing elements disposed at a surface of said at least onesheet of dielectric material, said References Cited by the Apphcam atleast one sheet of a dielectric material being dis- 15 UNITED STATESPATENTS posed between said at least one sheet of arbitrarily 3,176,2282/1965 Phillips thick but uniformly thick ceramic magnetic material andsaid energy absorbing elements in sandwich CHESTER L JUSTUS PrimaryExaminen fashion and said energy absorbing elements being most proximateto the radiator of electromagnetic 20 FISHER, Assistant Examiner-

1. A COMPOSITE MECHANICAL STRUCTURE DISPOSED TO COOPERATE WITH ARADIATOR OF ELECTROMAGNETIC ENERGY AND TO EFFECT ELECTROMAGNETIC ENERGYTRANSMISSION THROUGH THE COMPOSITE MECHANIAL STRUCTURE AND SUBSEQUENTLYTHROUGH A JUXTAPOSITIONED DENSE PLASMA MEDIUM WITH A MINIMUM OFATTENUATION OF ELECTROMAGNETIC ENERGY AND TO ABSORB ELECTROMAGNETICENERGY REFLECTED FROM A REFLECTING MEDIUM AT A DISTANCE FROM THECOMPOSITE MECHANICAL STRUCTURE COMPRISING: (A) AT LEAST ONE SHEET OFCERAMIC MAGNETIC MATERIAL; (B) AT LEAST ONE SHEET OF A DIELECTRICMATERIAL AFFIXED TO SAID SHEET OF CERAMIC MAGNETIC MATERIAL; AND, (C)ENERGY ABSORBING ELEMENTS DISPOSED AT SAID AT LEAST ONE SHEET OFDIELECTRIC MATERIAL SAID AT LEAST ONE SHEET OF A DIELECTRIC MATERIALBEING INTERPOSED BETWEEN SAID CERAMIC MAGNETIC MATERIAL AND SAID ENERGYABSRBING ELEMENTS IN SANDWICH FASHION.